The present invention relates to data storage media. More particularly, the present invention relates to the self-servowriting of servo patterns on storage media.
Self-servowriting has become an attractive technique for producing servopatterns on disk files because it eliminates the need for costly, external positioning systems, and can be performed outside of a clean room environment. In general, this technique involves using the read and write elements installed on the actuator of the disk drive xe2x80x9cin-situxe2x80x9d to initially write the servo pattern, which is thereafter used to correctly position the actuator during drive operation by a user.
Techniques for self-propagation of both radial positioning servo patterns and circumferential timing patterns have been developed recently. For example, in commonly assigned U.S. Pat. No. 5,659,436 entitled xe2x80x9cRadial Self-Propagation Pattern Generation for Disk File Servowritingxe2x80x9d (incorporated by reference herein in its entirety) the servo position signal used to control the head location while writing the next servo track is derived from the readback amplitude of a single track written one step earlier. In modern disk files, however, the read element can be offset from the write element on the actuator by as much as several tracks. When this read-to-write element offset gets large, it becomes desirable to use a combination of readback amplitudes from several earlier-written tracks to provide the position signal for the next track as described in commonly assigned U.S. Pat. No. 5,757,574 (incorporated by reference herein in its entirety) entitled xe2x80x9cMethods and Systems for Self-Servowriting Including Maintaining a Reference Level Within a Usable Dynamic Range.xe2x80x9d In such a case, the track immediately preceding the track being written is not reachable because of the offset between the read and write elements. A result of this process is that a given track contributes to the servo position determination on several subsequent tracks, each time with a different weight factor applied to its readback amplitude, in a weighted sum relationship.
A key challenge in radial self-propagation is the control of track shape error growth as the actuator is stepped across the disk surface to write the sequential servo-pattern tracks. The techniques described above compensate for the read-write element offset, however, a method for controlling track shape error growth has not been proposed.
In accordance with the present invention, a technique is disclosed for controlling error growth as the servowriting steps along the storage medium, along with a multitrack positioning technique.
In that regard, the present invention, in a first aspect, is a method for servowriting on a data storage medium in which a center of a write element is separated from a center of a read element along a direction in which the servowriting steps. One or more bursts are written on one track of the storage medium while servoing using a position signal derived from respective readback amplitudes of other bursts written previously on a plurality of tracks. A reference waveform is derived as a function of a position error waveform. The position error waveform corresponds to one or more position errors of the read element relative to the other bursts. The reference waveform is used when writing subsequent tracks on the storage medium when the read elements overlaps the one track.
The reference waveform is derived, in one embodiment, by calculating at least one complex coefficient of a discrete fourier transform of the position waveform; multiplying the complex coefficient by a complex filter factor f, thereby creating at least one filtered coefficient; calculating an inverse discrete fourier transfer of the at least one filtered coefficient; and adding the inverse discrete fourier transform to a nominal average reference level to form the reference waveform. The filter factor f can be computed from a predetermined function of a closed loop response C of a servoloop used for the servowriting. Multiple reference waveforms from respective tracks are combined in a weighted sum, the weights of which are calculated according to the relative sensitivity of the position signal to shifts in the position of each of the tracks respectively.
In another aspect of the present invention, which can be used in combination with the first aspect disclosed above, or separately, a method for servowriting on a data storage medium is provided in which a center of a write element is at least partially separated from a center of the read element along a direction in which the servowriting steps. In this multitrack positioning embodiment, one or more bursts are written on one track of the storage medium while servoing using a position signal derived from respective readback amplitudes of other bursts written previously on a plurality of tracks. In this embodiment, the position signal is derived using a parabolic interpolation of the readback amplitudes of the other bursts written previously on the plurality of tracks.
In one embodiment, three tracks are used in the parabolic interpolation, the center of which has the highest readback amplitude, a track preceding the center track having a lower amplitude, and a track subsequent to the center track, having another, lower amplitude. A specific form of the parabolic interpolation function is also disclosed herein.